This invention relates to emission control for combustion apparatus burning carbonaceous fuels and, more particularly, to compositions and methods for the enhanced removal of particulates, hazardous substances, nitrogen oxides, and sulfur oxides from a flue gas stream resulting from the combustion of these fuels.
Environmental regulations require that emissions of certain materials in flue gases be kept at levels not exceeding those set forth in federal, state, and local specifications. To comply with these legal mandates, particulate emissions must satisfy certain standards in terms of pounds per million Btu input, pounds per unit time, and opacity of stack effluent. The term "particulate" within the meaning of these restrictions generally refers to fly ash and other fine particles found in flue gas streams and can include a host of hazardous substances, such as those listed in 40 CFR .sctn. 302.4 (e.g., arsenic, ammonia, ammonium sulfite, mercury, and the like).
Acidic gases are also found in flue gas streams. Whenever sulfur-containing fuels are burned, sulfur is converted to sulfur dioxide and sulfur trioxide (together known as "SO.sub.x ") and released into the atmosphere along with other flue gases and entrained particulate and hazardous substance materials. Combustion of carbonaceous fuels also results in the formation of nitric oxide and nitrogen dioxide (together known as "NO.sub.x "), which also exit the stack with the combustion exhaust materials. However, as in the case of particulates, the emissions of both NO.sub.x and SO.sub.x are subject to certain output standards because of acid rain legislation and mandatory ambient air quality criteria. Therefore, at least with respect to SO.sub.x, one is required to burn low-sulfur fuels to ensure compliance with SO.sub.x emission requirements. This adversely affects older emission control devices that were originally designed to work in units burning higher-sulfur fuels. There are also enormous costs associated with transporting low-sulfur fuels to locations where such fuels are not found in abundance.
The method of improving particulate control known as flue gas conditioning is generally understood as adding a chemical into the flue gas streams of boilers, turbines, incinerators, and furnaces to improve the performance of downstream emission control devices. Although the term is usually associated with the removal of particulates caused by coal combustion, flue gas conditioning can be equally effective in controlling particulates caused by the burning of any carbonaceous fuel. As this invention illustrates, flue gas conditioning can also be used to remove hazardous substances, NO.sub.x, and SO.sub.x from the flue gas stream.
The performance of downstream emission control devices, such as electrostatic precipitators, often depends upon the chemistry of the flue gases and, in particular, such factors as the fuel sulfur content, particulate composition, particulate resistivity, and the cohesion properties of entrained particulates, to name a few. Chemical additives either to the fuel prior to combustion or to the flue gas stream prior to the electrostatic precipitator can correct the deficiencies of the precipitator to meet particulate emissions standards (e.g., mass emission and visual opacity). One of the objects of flue gas conditioning is to enhance the effectiveness of the electrostatic precipitation process by manipulating the chemical properties of the materials found in the flue gas stream.
Gases, such as ammonia and sulfur trioxide, when injected into the flue gas stream prior to a cold-side electrostatic precipitator, have been known to condition the fly ash for better precipitator performance. Similar results have been obtained with inorganic chemical compounds, such as ammonium sulfate, sodium bisulfate, sodium phosphate, or ammonium phosphate. The use of sulfuric acid has also been proposed, as well as mixtures of these inorganic compounds in the form of undisclosed "proprietary blends." These compounds have been added either as a powder or as an aqueous solution to the flue gas stream.
Organic compounds, such as ethanol amine and ethanol amine phosphate, have also been used as flue gas conditioning agents. Free-base amino alcohols, such as morpholine (including morpholine derivatives), have been used as well to augment the flow characteristics of treated fly ash. Similarly, the use of alkylamine (such as tri-n-propylamine) and an acid containing sulfur trioxide (such as sulfamic acid) has been proposed to lower the resistivity of fly ash.
Anionic polymers have been employed in situations where the fly ash resistivity needs to be lowered, particularly when a low-sulfur coal is utilized. Similarly, cationic polymers have been suggested whenever the electrical resistivity needs to be raised from a low value, such as when using high-sulfur coal. Anionic polymers containing ammonium and sodium nitrate have also been known to increase the porosity of fly ash for principal application in bag houses.
The use of inorganic salts, such as sodium sulfate, sodium carbonate, or sodium bicarbonate added directly to the coal before combustion has been known to correct the "sodium depletion" problems of a hot-side precipitator. Sodium carbonate and sodium bicarbonate have also been injected directly into the flue gas stream prior to the hot-side precipitator, but this mode of application has not been commercialized.
The principal post-combustion method for controlling S.sub.x emissions involves the saturation of basic chemicals with the flue gases through the use of a "scrubber." In this removal method, advantage is taken from the fact that SO.sub.x is acidic in nature and will react with basic additives to form an innocuous sulfate. Essentially, the principle underlying the various forms of scrubber technologies is to utilize simple acid-base reactions to control SO.sub.x emissions. However, conventional scrubber designs are very capital intensive to build and remain expensive to operate in terms of labor, energy, and raw material costs.
There are many types of scrubbers currently in use. In wet scrubbers (which are normally located after the emission control device), the flue gas is brought into direct contact with a scrubbing fluid that is composed of water and a basic chemical such as limestone (calcium carbonate), lime, caustic soda, soda ash, and magnesium hydroxide/carbonate, or mixtures of these. Water-soluble nitrite salts have also been added to the scrubbing medium for the purpose of enhancing the SO.sub.x -removal efficiency of wet scrubbers. The use of organo phosphonic acid in conjunction with water-based solutions or slurries that react with sulfur dioxide have been known to improve the utilization of the basic material in a wet scrubber. Similarly, polyethylene oxide compounds have been added to the flue gas as a sludge de-watering agent for improving the wet scrubber's efficiency.
In dry scrubbers, slurries of lime or mixtures containing lime and other basic chemicals are injected into the flue gas stream as sprays. Unlike the wet scrubbers, the injection of these chemicals in dry scrubbers is usually conducted before the emission control device. After injection, the unreacted chemicals and reaction products become entrained with the flue gas stream and are separated from the flue gas along with other particulates in the downstream emission control device using common particulate removal techniques. However, a problem encountered with this method of SO.sub.x removal is that the unreacted chemicals and reaction products cause a very heavy particulate load on the downstream emission control device. This method of removal is also less efficient than wet scrubbing techniques due to the low reaction rates between sulfur dioxide and the dry scrubbing additives.
Because of its very high reaction rate with sulfur dioxide, a compound known as "trona" (a hydrous acid sodium carbonate) has also been injected into the flue gas stream in dry scrubbers (upstream from the emission control device) in an effort to reduce SO.sub.x emissions. Unfortunately, trona produces an undesirable side effect--it provokes NO.sub.2 formation, which is another pollutant that is very visible in the plume by its characteristic brown, aesthetically unacceptable color. Notwithstanding its low cost, therefore, trona has not acquired much popularity.
The use of soda ash (anhydrous sodium carbonate), caustic soda (sodium hydroxide), and calcium hydroxide in dry and wet scrubbers has also proven effective in reducing SO.sub.x emissions. However, these strong bases have achieved limited commercial success because of high raw material costs. For example, 1.25 tons of caustic soda is required for removing every ton of sulfur dioxide produced. For a 500-megawatt power station burning 2% sulfur coal, it would require 270 tons of soda per day to keep SO.sub.x emissions within acceptable levels.
As mentioned previously, NO.sub.x is also produced during the combustion of carbonaceous fuels. NO.sub.x is generated by several means, such as the fixation of nitrogen present in combustion gases, the conversion of fuel-derived nitrogen, and prompt NO.sub.x formation. Prompt NO.sub.x formation is a small contributor and only occurs under very fuel-rich operations.
There are several methods by which NO.sub.x emissions have been controlled. One of these methods include the injection of ammonia directly into the combustion chamber. Maintaining a close temperature control between 1650.degree. F. to 1832.degree. F. is essential under this technique; otherwise, the desired NO.sub.x removal will not occur, and there will be an excessive emission of unreacted ammonia. Excessive emissions of unreacted ammonia from the combustion chamber (known as "ammonia slippage") not only adds to pollution but also causes pluggage of downstream equipment. Ammonia slippage thus becomes a problem in its own right.
In another method for NO.sub.x removal, known as "SCR" or selective catalytic reduction, ammonia is added to the flue gas stream at temperatures above 800.degree. F. The mixed stream is then passed over a catalyst where the NO.sub.x removal process is effected. Despite being the most expensive technology, based both on initial capital and operating costs, this method has provided the best removal rates of NO.sub.x (removal rates of 90% to 99% are common). Unfortunately, however, the catalysts are subject to degradation over time, as well as poisoning by sulfur-containing gases and poisoning and blinding by fly ash.
In yet another method, known as "SNCR" or selective non-catalytic reduction, urea (or its precursors) is injected into the flue gas stream at temperatures between 1600.degree. F. to 1800.degree. F. As in the case of the ammonia-injection method for NO.sub.x control, however, the SNCR process must operate in a narrow temperature window or else ammonia slippage will occur or too little NO.sub.x reduction will be achieved. Although combinations of SNCR and SCR have been proposed, they have presented similar limitations.
Unlike the aforementioned emission control methods, use of the compositions of the present invention provides an effective, efficient, and low-cost means for controlling particulate, hazardous substance, NO.sub.x, and SO.sub.x emissions without exhibiting any of the above limitations. Moreover, use of the invention compositions fills an important need by reducing these emissions simultaneously. Because of these desirable characteristics, the present invention constitutes a significant advancement over prior emission control techniques.